Packaging containers of the disposable type for liquid foods are often produced from a packaging material based on paperboard or carton. One such commonly occurring packaging container is marketed under the trademark Tetra Brik® Aseptic and is principally employed for aseptic packaging of liquid foods such as milk, fruit juices etc, marketed and sold for long term ambient storage. The packaging material in this known packaging container is typically a laminate comprising a bulk layer of paper or paperboard and outer, liquid-tight layers of thermoplastics. In order to render the packaging container gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging such as packaging of milk or fruit juice, the packaging material laminate in these packaging containers normally comprises at least one additional layer, providing gas barrier properties, most commonly an aluminium foil.
On the inside of the laminate, i.e. the side intended to face the filled food contents of a container produced from the laminate, there is an innermost layer, applied onto the aluminium foil, which innermost, inside layer may be composed of one or several part layers, comprising heat-sealable adhesive polymers and/or heat-sealable polyolefins. Also on the outside of the paper or paperboard bulk layer, there is an outermost heat-sealable polymer layer (decor layer). The heat-sealable polymer layers are often based on low density polyethylene or blends thereof.
The packaging containers are generally produced by means of high-speed packaging machines of the type that continuously form, fill and seal packages from a web of packaging material, e.g. Tetra Brik® Aseptic-type packaging machines. Packaging containers may thus be produced by the so-called form-fill-seal technology basically including continuous forming a web into a tube of the laminated packaging material by means of a filling machine which forms, fills and seals the web into packages. According to another method, packages are made one by one from prefabricated blanks of packaging material, e.g. in Tetra Rex®-type filing machines, thus providing so-called gable-top shaped packages, or other blanks-based packages.
An aspect of the continuous Tetra Brik®-type packaging method is, as stated above, the possibility of high-speed packaging, which has considerable impact on cost efficiency. Typically, many thousands of packages may be prepared per hour. For example the Tetra Pak® A3/speed may manufacture about 15 000 packages per hour (family-size packaging containers of 0.9 liters and above), and about 24 000 packaging containers per hour (portion packages).
There have been attempts to modify a paperboard laminate by replacing the paperboard bulk layer by a polymer foam, as for example described in WO00/30846. Such attempts did, however, not come to practical products, because foams from cost-efficient polymers normally deteriorate and collapse when exposed to high temperature and pressure during the conversion of the materials into a laminated material. This is a particular problem during polymer melt extrusion lamination operations, wherein polymer foams collapse by the heat of the molten polymer and the pressure in the lamination nip, but also during lamination steps, in which layers having pre-coated surfaces of a thermoplastic polymer or adhesive, are brought to adhere to each other by simultaneous application of heat and pressure in a heated roller nip, i.e. so-called rotational heat-pressure lamination.